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Author Topic: 12at7 PI  (Read 6085 times)

sgmypod

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12at7 PI
« on: April 23, 2011, 04:49:28 PM »
What difference to sound and feel does it make changing a 12ax7 to 12at7 in phase invertor position....obviously will change the gain, but what else will it do?
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djl

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Re: 12at7 PI
« Reply #1 on: April 23, 2011, 05:17:55 PM »
It depends quite a lot on the amp, as far as I know. In some amps, like vintage fenders, the PI position has quite a dramatic effect on the sound of the amp. I think in most other amps, like most modern ones, the effect of changing the PI valve will have quite a small effect - nowhere near as swapping V1 for example.

Generally 12at7s are lower output, as you say, and also sound a bit more "brittle". Some people prefer using the 5751 (a lower gain 12ax7) if they want to lower the gain of an amp - SRV is supposed to have used one in his amps. Not sure if he used it in the PI position or V1 though.

Like all these things, there's only one way to find out... You shouldn't blow your amp up changing preamp valves around, as far as I know. 12at7s do draw more current though

Dmoney

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Re: 12at7 PI
« Reply #2 on: April 23, 2011, 07:37:20 PM »
I've experimented with swapping valves in the PI.
you do reduce the voltage of the signal going into the output valve grids so it makes the amp quieter than when using a 12AX7 at the same point on the volume dial. i think the tonal difference would be down to the valve specifically. I have a Tungsram 12AU7 that I use in the little high gain 6V6 amp I made to tame the level into the 6V6's. I used to have an 12AT7 but I don't know where it went. compared to the AX7 the AU7 is a bit mushy, but I imagine you'd get a different result with a different 12AU7. you know how much 12AX7s vary so the same applies. the AU7 has a lot less gain than the AT7 though. so AT7 vs AX7 isn't as big of a jump.

It is a noticeable difference in tone as well as output level. while the voltage level is reduced, the AT7 can provide more current in its output which is beneficial in some electrical situations.

I think the difference is going to depending largely on what your amp is, how your play, and how you play with the AT7. It'll just be a bit different. you might like it.

The 12AT7 is also worth trying in the middle of the preamp if you want to reduce gain. It can have some pretty dramatic effects even in a really high gain head.
 

« Last Edit: April 23, 2011, 08:10:08 PM by Dmoney »

Transcend

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Re: 12at7 PI
« Reply #3 on: April 23, 2011, 07:47:46 PM »
great description by Dmoney there and i found an article regarding this earlier which is extremely informative
and an extremely interesting read for myself.

http://www.guitaramplifierblueprinting.com/files/Phaseinverter.pdf

Tellboy

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Re: 12at7 PI
« Reply #4 on: April 24, 2011, 08:52:06 AM »
great description by Dmoney there and i found an article regarding this earlier which is extremely informative
and an extremely interesting read for myself.

http://www.guitaramplifierblueprinting.com/files/Phaseinverter.pdf

Good read - he obviously is not a fan of channel switching amps  :D
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Dmoney

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Re: 12at7 PI
« Reply #5 on: April 24, 2011, 10:08:03 AM »
hahah guess not!
The first 3 paragraphs seemed mainly like the guys own opinion/preference.
He also misses a point on master volumes too, that with a PPIMV you can drive the PI way harder than with a pre-PIMV
AND
not all amps use a long-tail pair phase inverter although it is the most common, and all the different PI circuits distort differently. For example, the Vox night train has a Cathodyne PI. So It's not weird and wacky amps that differ in that part of the circuit, these things are still common.




Frank

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Re: 12at7 PI
« Reply #6 on: April 24, 2011, 10:13:09 AM »
That PDF has probably become the most widely quoted and linked article on the internet about the subject but he makes some fairly strange claims about tubes and amps in it! Output current 10mA? He's quoting the plate current from the spec sheet, 10 mA plate current does not equal 10mA output current to the grids of the output tubes. Especially as he claims master volume amps have a master volume after the PI (do they? news to me).

Dmoney

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Re: 12at7 PI
« Reply #7 on: April 24, 2011, 10:25:54 AM »
That PDF has probably become the most widely quoted and linked article on the internet about the subject but he makes some fairly strange claims about tubes and amps in it! Output current 10mA? He's quoting the plate current from the spec sheet, 10 mA plate current does not equal 10mA output current to the grids of the output tubes. Especially as he claims master volume amps have a master volume after the PI (do they? news to me).

nah he mentions both types of master volume but treats them the same, which isn't the best idea when talking about the subject of how a valve is going to react in the PI. I assume he means master volume amps can have either rather than both. I guess you could have both but I dunno why you would want that!
I think there is a fair bit of bias and assumption in it. haha.


Frank

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Re: 12at7 PI
« Reply #8 on: April 24, 2011, 10:32:05 AM »
It is a noticeable difference in tone as well as output level. while the voltage level is reduced, the AT7 can provide more current in its output which is beneficial in some electrical situations.

It's more a matter of output impedance - 12AT7 has much lower plate resistance, therefore lower output impedance and lower Zout means more high frequency response.

Transcend

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Re: 12at7 PI
« Reply #9 on: April 24, 2011, 11:04:05 AM »
Upon re reading today when im a lot more awake i can actually see all the bias in the article.

However as with any piece of refererance material that is all it is.

The only way to find out if it works for you & your sound is to try it yourself.


Frank

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Re: 12at7 PI
« Reply #10 on: April 24, 2011, 11:06:31 AM »
The only way to find out if it works for you & your sound is to try it yourself.

I think that's how most amp designers do it anyway :)

Dmoney

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Re: 12at7 PI
« Reply #11 on: April 24, 2011, 11:16:59 AM »
The only way to find out if it works for you & your sound is to try it yourself.

I think that's how most amp designers do it anyway :)

I thought a lot just copied marshall or fender.
Apart from high F response (thanks for the explanation by the way) isn't their a benefit where by blocking distortion is decreased if you're driving you amp hard while using a AT or AU over an AX? I can't remember why I think that.


EDIT: Also the lower plate resistance means that they bias up more... naturally (couldn't really think of a good word) than a 12AX7 when used as a cathode follower? Which changes the tone that you get from the normal "poor" design of a cathode follower.
« Last Edit: April 24, 2011, 11:26:29 AM by Dmoney »

Frank

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Re: 12at7 PI
« Reply #12 on: April 24, 2011, 11:50:30 AM »
Well some people say that any cathode follower using a 12AX7 is a poorly designed cathode follower!

The main advantage of a CF is high input impedance and low output impedance. That means it doesn't "load" the input signal too heavily and it can deliver voltage/current into a wide range of load resistances. That's why they're handy for driving effects loops and tone stacks. Using a 12AX7 as a CF just loses some of the low Zout advantage due to the higher plate resistance.

However, as you say, most people copy the Fender and Marshall circuits. And Jim Marshall copied Fender. And Leo Fender copied everything out of the Radio Designers Handbook and tube manufacturer's data sheets. And the amps still sound good, even if they sometimes use "bad" design.

Really a lot of the "classic" sounds of old amps are due to the fact that Leo was working to produce cheap amps. If he'd tried to build high-fidelity amps then we'd have ultralinear output stages, expensive speakers that produced more top end response and preamps with enough headroom to reproduce the guitar sound with no distortion. And we'd never have rock music as we know it.

sgmypod

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Re: 12at7 PI
« Reply #13 on: April 24, 2011, 11:58:56 AM »
Lol had read the article before posting, thanks for posts will try this week
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HTH AMPS

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Re: 12at7 PI
« Reply #14 on: April 26, 2011, 05:51:33 PM »
The only way to find out if it works for you & your sound is to try it yourself.

I think that's how most amp designers do it anyway :)

shhh!!!!, don't be giving the secrets away  :lol: